1. Field of the Invention
The present invention relates to a multicylinder spark-ignition engine of which individual cylinders undergo successive combustion cycles with specific phase delays.
2. Description of the Related Art
There exist conventionally known approaches to improving fuel economy of a spark-ignition engine. One approach to fuel economy improvement is to burn a mixture at an air-fuel ratio higher than the stoichiometric air-fuel ratio in individual cylinders of a spark-ignition engine. An example of this approach is disclosed in Japanese Unexamined Patent Publication No. 1998-29836. The approach of the Publication employs fuel injectors for injecting fuel directly into combustion chambers to produce stratified charge combustion by injecting the fuel during a compression stroke when the engine is in a low-load, low-speed range, for instance, so that the mixture is burned under extremely lean conditions.
In this type of engine, it is impossible to sufficiently convert nitrogen oxides (NOx) under lean burn operating conditions by using as an emission-cleaning converter an ordinary three-way catalyst alone, which is a catalyst having high emission-cleaning performance to convert hydrocarbons (HC), carbon monoxide (CO) and NOx at about the stoichiometric air-fuel ratio. Therefore, as mentioned in Japanese Unexamined Patent Publication No. 1998-29836, the engine is provided with a lean NOx catalyst which adsorbs NOx in an oxygen-rich atmosphere and releases and reduces NOx in a decreased oxygen concentration atmosphere. If the amount of NOx adsorbed by the lean NOx catalyst has increased under the lean burn operating conditions when the lean NOx catalyst of this kind is being used, the fuel is injected not only for primary combustion but an additional amount of fuel is injected during an expansion stroke to lower the air-fuel ratio and generate CO for accelerating release and reduction of NOx as shown in the aforementioned Publication, for example.
In the aforementioned conventional engine which performs lean burn operation, it is necessary to provide an expensive lean NOx catalyst in an exhaust passage to realize high NOx-converting performance under the lean burn operating conditions. The provision of the lean NOx catalyst is disadvantageous from the viewpoint of product cost. Furthermore, it is necessary to temporarily lower the air-fuel ratio by supplying an additional amount of fuel to accelerate release and reduction of NOx when the amount of adsorbed NOx increases as stated above in order to maintain the converting performance of the lean NOx catalyst. Moreover, the lean NOx catalyst is susceptible to poisoning by sulfurization if the fuel used has a high sulfur content. The lean NOx catalyst should therefore be subjected to a regeneration treatment, such as heating of the catalyst and feeding of a reducing agent, to overcome this sulfur-poisoning problem of the lean NOx catalyst. All such problems of the conventional engine would jeopardize fuel economy improvement effects of the lean burn operation.
Another approach to fuel economy improvement is compression ignition which has been intensively studied in recent years. The compression ignition is spontaneous firing of a mixture in a combustion chamber occurring under high temperature and pressure conditions in a final part of a compression stroke as in a diesel engine. When the compression ignition is produced, the mixture rapidly burns throughout the entire combustion chamber even under conditions where the air-fuel ratio in the combustion chamber is extremely high or a large amount of burned gas is introduced into the combustion chamber by exhaust gas recirculation (EGR). This makes it possible to prevent too late combustion which does not produce any effective work. The compression ignition approach is therefore advantageous for improving the fuel economy.
In an ordinary spark-ignition engine, however, the temperature and pressure in the combustion chamber do not increase to such levels that are high enough to produce compression ignition at about a top dead center in the compression stroke. To produce compression ignition, it is necessary to make a special arrangement to considerably increase the temperature or the pressure in the combustion chamber. It has conventionally been difficult, however, to increase the temperature or the pressure in the combustion chamber to a level high enough to produce compression ignition in a part-load range in which the fuel economy must be improved while avoiding knocking (abnormal combustion caused by spontaneous firing of the mixture occurring before propagation of a flame in the combustion chamber) in a high-load range.
Under such circumstances, the Applicant previously filed Japanese Patent Application No. 2002-185242 for technology concerning a control device for a spark-ignition engine, which is intended to achieve a significant improvement in fuel economy by a combination of the lean burn operation and compression ignition. According to the claimed technology, a special operation mode is selected in a part-load range of the engine, in which a pair of preceding and following cylinders of which exhaust and intake strokes overlap is connected to form a two-cylinder interconnect configuration. When the two-cylinder interconnect configuration is established, burned gas discharged from the preceding cylinder which is currently in the exhaust stroke is introduced into the following cylinder which is currently in the intake stroke through an intercylinder gas channel, wherein a lean mixture having an air-fuel ratio higher than the stoichiometric air-fuel ratio is combusted by forced ignition in the preceding cylinder, whereas fuel is supplied to the burned gas having a high air-fuel ratio introduced from the preceding cylinder into the following cylinder so that a mixture thus produced in the following cylinder is combusted by compression ignition.
If the engine is controlled in the aforementioned special operation mode in the part-load range, a substantial fuel economy improvement effect is obtained in the preceding cylinder as a result of an improvement in thermal efficiency achieved by the lean burn operation and a reduction in pumping loss in the preceding cylinder. In the following cylinder, on the other hand, the mixture produced by supplying the fuel to the burned gas having a high air-fuel ratio introduced from the preceding cylinder is combusted. As the air-fuel ratio in the following cylinder is made equal to the stoichiometric air-fuel ratio in the special operation mode, pumping loss in the following cylinder is reduced and this produces a further fuel economy improvement effect.
In addition, since the high-temperature burned gas is introduced from the preceding cylinder into the following cylinder through the intercylinder gas channel, compression ignition occurs in a combustion chamber of the following cylinder after the temperature in the combustion chamber has increased up to a level high enough to produce compression ignition in the compression stroke. This serves to significantly improve the fuel economy and to decrease the amount of NOx emissions. Moreover, since the following cylinder discharges only such burned gas that is produced by burning the mixture at the stoichiometric air-fuel ratio, it is possible to realize satisfactory emission-cleaning performance with the three-way catalyst alone without the need for the provision of the lean NOx catalyst in an exhaust passage.
The spark-ignition engine of Japanese Patent Application No. 2002-185242 is made switchable depending on engine operating conditions between a normal operation mode in which combustion is produced independently in individual cylinders which are connected to form an independent cylinder configuration and the aforementioned special operation mode in which combustion is produced in the two-cylinder interconnect configuration with the burned gas discharged from the preceding cylinder introduced directly into the following cylinder. In this spark-ignition engine, a problem tends to occur in control operation for switching gas flow paths from the independent cylinder configuration to the two-cylinder interconnect configuration. The emission-cleaning performance of the engine could deteriorate during a transition period of this gas flow path switching operation. More particularly, it is so difficult to instantly alter and regulate the quantity of fresh intake air immediately following the gas flow path switching operation, and this poses a problem in controlling the air-fuel ratio which is the ratio of the quantity of fresh intake air to the quantity of fuel (gasoline). This makes it difficult to clean exhaust gases by using the three-way catalyst alone which has the high emission-cleaning performance at about the stoichiometric air-fuel ratio. Therefore, the emission-cleaning performance potentially deteriorate in the aforementioned spark-ignition engine.